Spot the bug: Unmanaged memory traps

time to read 4 min | 769 words

Sometimes I forgot how good life is in the managed lane.  Then I do some unmanaged work and get a good reality check.

Let us look at the following structures:

    [StructLayout(LayoutKind.Explicit, Pack = 1)]
    public struct PageHeader
        [FieldOffset(0)] public long Marker;

        [FieldOffset(8)] public ushort Lower;

        [FieldOffset(10)] public ushort Upper;

        [FieldOffset(12)] public int OverflowSize;

        [FieldOffset(16)] public int ItemCount;

    [StructLayout(LayoutKind.Explicit, Pack = 1)]
    public struct FileHeader
        [FieldOffset(0)] public long Marker;

        [FieldOffset(8)] public LogHeader Active;

        [FieldOffset(44)] public LogHeader Backup;

        [FieldOffset(80)] public TreeRootHeader Root;

        [FieldOffset(142)] public TreeRootHeader FreeSpace;

    [StructLayout(LayoutKind.Explicit, Pack = 1)]
    public struct TreeRootHeader
        [FieldOffset(0)] public long RootPageNumber;
        [FieldOffset(8)] public long BranchPages;
        [FieldOffset(16)] public long LeafPages;
        [FieldOffset(32)] public long OverflowPages;
        [FieldOffset(40)] public long PageCount;
        [FieldOffset(48)] public long EntriesCount;
        [FieldOffset(56)] public int Depth;
        [FieldOffset(60)] public TreeFlags Flags;

    [StructLayout(LayoutKind.Explicit, Pack = 1)]
    public struct LogHeader
        [FieldOffset(0)] public long Marker;

        [FieldOffset(8)] public long LastLog;

        [FieldOffset(16)] public long LastLogPage;

        [FieldOffset(24)] public int ItemCount;

        [FieldOffset(28)] public long Options;

And now we have the following code:

   1:  private static unsafe void Main()
   2:  {
   3:      IntPtr pagePtr = Marshal.AllocHGlobal(4096);
   5:      var pageHeader = (PageHeader*) pagePtr.ToPointer();
   6:      pageHeader->ItemCount = 2;
   7:      pageHeader->Marker = 0x128314543423;
   8:      pageHeader->OverflowSize = 32;
  10:      FileHeader* fileHeader = (FileHeader*) pageHeader + sizeof (PageHeader);
  12:      fileHeader->Root.BranchPages = 0;
  15:      Marshal.FreeHGlobal(pagePtr);
  16:  }

The fun part about this code is that it would silently corrupt the state of the process.

Here is what happens when you run it under the debugger:


Can you figure out why?